JPH0989838A - Gas sensor and gas detection method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance the reliability and durability of a sensor for detecting the concentration of carbon monoxide in the exhaust gas from a burner. SOLUTION: The concentration of carbon monoxide is detected based on the electromotive force induced between an electrode 2 covered with a carbon monoxide oxidation catalyst 1 and an electrode of solid electrolyte 4 not covered with carbon monoxide oxidation catalyst 1. An interfering gas is prevented from intruding into a detecting section by covering the solid electrolyte 4 with a membrane 5 for passing the gas selectively and the film 5 is cleaned by a heating means 6. Furthermore, the sensor can be self-diagnosed by supplying a voltage between the electrodes from a voltage supply and measuring the current.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in reliability and durability of a carbon monoxide sensor used for air-fuel ratio control of combustion equipment, incomplete combustion alarm and the like.

[0002]

2. Description of the Related Art A conventional carbon monoxide sensor using a solid electrolyte is disclosed in, for example, Japanese Patent Application Laid-Open No. 59-10985 shown in FIG.
As described in Japanese Patent Publication No. 6, a solid ceramic body 4 having electrodes 2 and 3 on both sides is put in a cylindrical body 20 of a porous ceramic to form a partition wall, and a flammable gas oxidation catalyst 21 is provided on one side of the partition wall. The particles are filled on the other side with particles of an oxidation catalyst 22 for a combustible gas other than carbon monoxide, and the particles are sealed with a porous ceramic plate 23, and the porous ceramic cylinder 20 is heated from the outside by a heating means 6
The heating is performed by the method described above, and it is intended to detect carbon monoxide that has passed through the porous wall surface and reached the solid electrolyte body 4 inside.

[0003]

However, in the above-mentioned conventional configuration, when the gas sensor is installed in the combustion exhaust gas, not only carbon monoxide but also an organic substance having a large molecular weight, nitrogen oxides or the like is detected in the carbon monoxide sensor. The catalyst and the electrode portion reach through the pores of the porous ceramic. As a result, there are problems that the catalyst and the electrodes are contaminated and the carbon monoxide detection characteristics are deteriorated. Further, even if the gas sensor deteriorates, the gas sensor does not have a means for judging its own characteristic deterioration.

The present invention is intended to improve the reliability by preventing the deterioration of the characteristics of the carbon monoxide gas sensor, improving the durability of the carbon monoxide sensor, and having a means for judging the deterioration of the sensor itself. It is intended.

[0005]

In order to achieve the above object, the present invention provides oxygen having a pair of electrodes, a catalyst-covered electrode and a non-catalyst-covered electrode having an ability to oxidize carbon monoxide. A solid electrolyte body having ionic conductivity, a porous gas selective permeable membrane having a pore diameter of 100 angstroms or less covering the solid electrolyte body, and a heating means in the vicinity of the gas selective permeable membrane. .

Further, a solid electrolyte body having oxygen ion conductivity having electrodes on the front and back surfaces and a pair of electrodes are covered.
A porous gas selective permeable membrane having a pore diameter of 0 angstroms or less, and a catalyst having the ability to oxidize carbon monoxide in the pores of the gas selective permeable membrane facing one of a pair of electrodes. It is configured to be supported and provided with heating means in the vicinity of the gas selective permeable membrane.

Further, a porous gas selective permeable membrane formed into a hollow columnar shape with a controlled pore diameter of 100 angstroms or less, an electrode covered with a catalyst having an ability to oxidize carbon monoxide, and an electrode not covered with a catalyst. A solid electrolyte body having oxygen ion conductivity and having a pair of electrodes arranged so that both ends of a hollow cylindrical gas selective permeable membrane are sealed, and a coiled heater is provided on the outer peripheral portion of the gas selective permeable membrane. Is.

Further, a solid electrolyte body having oxygen ion conductivity, in which a pair of electrodes, that is, an electrode covered with a catalyst having an ability to oxidize carbon monoxide and an electrode not covered with a catalyst, is arranged,
A gas sensor comprising a porous gas selective permeable membrane having a pore size controlled to 100 angstroms or less, which covers a solid electrolyte body, and a heating means in the vicinity of the gas selective permeable membrane, and an electromotive force measuring means between a pair of electrodes and an electrode. It is provided with a voltage applying means between the electrodes, a current measuring means for measuring a current between the electrodes, and a switching means for switching between the electromotive force measuring means and the current measuring means.

A solid electrolyte body having oxygen ion conductivity, in which a pair of electrodes, an electrode covered with a catalyst having an ability to oxidize carbon monoxide and an electrode not covered with a catalyst, is arranged,
A porous gas selective permeable membrane having a pore size controlled to 100 angstroms or less, covering the solid electrolyte body, a heating means near the selective permeable membrane, an electromotive force measuring means between a pair of electrodes, and a voltage applying means between the electrodes. And a current measuring means for measuring the current between the electrodes, a switching means for the electromotive force measuring means and the current measuring means, and a self-diagnosis means for judging an abnormality of the gas sensor from the magnitude of the measured current. is there.

Further, a solid electrolyte body having oxygen ion conductivity, in which a pair of electrodes, an electrode covered with a catalyst having an ability to oxidize carbon monoxide and an electrode not covered with a catalyst, is arranged,
A porous gas selective permeable membrane having a pore size controlled to 100 angstroms or less, which covers the solid electrolyte body, a heating means near the gas selective permeable membrane, and a heating temperature control means of the heating means.

[0011]

With the above-mentioned structure, the present invention serves to selectively filter an organic substance having a large molecular weight by a porous gas selective permeable membrane to prevent it from reaching a catalyst or an electrode. The heating means installed near the porous gas permselective membrane activates the carbon monoxide detection characteristic by heating the solid electrolyte body or the catalyst.

Further, the combustion gas that has passed through a catalyst having the ability to oxidize carbon monoxide in the pores of the gas selective permeable membrane and has reached one electrode, and the combustion gas that has not passed through the catalyst and has reached the other electrode This causes a potential difference between both electrodes.

The hollow columnar gas-selective permeable membrane selectively filters an organic substance having a large molecular weight and does not allow the organic substance having a large molecular weight to reach a solid electrolyte having an electrode installed in the hollow portion.

The electromotive force measuring means between the electrodes measures the electromotive force according to the concentration of carbon monoxide. On the other hand, the voltage applied between the electrodes by the voltage applying means moves oxygen ions in the solid electrolyte body having oxygen ion conductivity to diffuse oxygen from the pores of the gas selective permeable membrane into the atmosphere. At this time, the current flowing between both electrodes measured by the current measuring means shows a current value according to the degree of porosity of the gas selective permeable membrane and the oxygen concentration. Therefore, by switching the electromotive force measuring means and the current measuring means by the switching means and measuring the voltage value and the current value, both the carbon monoxide and oxygen detecting means can be obtained.

The magnitude of the current measured by the current measuring means for measuring the current between the electrodes is determined by the degree of porosity of the gas selective permeable membrane when the oxygen concentration is constant. When the current value is not within a certain range, the gas selective permeable membrane may be damaged,
Since the electrodes are peeled off, the failure diagnosis of the gas sensor can be performed by monitoring the current value.

The heating means installed in the vicinity of the gas selective permeable membrane maintains the operating temperature for sensitizing the carbon monoxide detection sensitivity by the heating temperature control means and burns out the contaminants of the gas selective permeable membrane. The temperature is controlled so that the sensor is cleaned.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS.

In FIG. 1, 1 is a carbon monoxide oxidation catalyst, 2 and 3 are electrodes formed by a pair of platinum vapor depositions, 4 is a solid electrolyte body, 5 is a gas selective permeable membrane, and 6 is a solid electrolyte body 4. Is a heating means for heating and to set a temperature environment that guarantees the operating characteristics of the gas sensor. Reference numeral 7 is a lead wire taken out from the electrodes 2 and 3.

If the combustion exhaust gas reaches the solid electrolyte body 4 when the gas sensor having the above-mentioned configuration is installed in the exhaust passage of the combustion equipment, the carbon monoxide in the exhaust gas is oxidized by the carbon monoxide oxidation catalyst 1. Oxygen is ionized by the reaction represented by (Chemical Formula 1) without reaching the electrode 2.

O ₂ + 2e ⁻ → O ^{2 −} (Chemical formula 1) On the other hand, in the electrode 3 not covered with the carbon monoxide oxidation catalyst 1, the reaction expressed by the (Chemical formula 1) and carbon monoxide reach the electrode. Therefore, two kinds of reactions represented by (Chemical Formula 2) occur.

CO + O ^{2 −} → CO ₂ + 2e ⁻ (Chemical formula 2) The electromotive force between the electrodes 2 and 3 is the potential difference between both electrodes, and the electromotive force increases as the amount of carbon monoxide increases. The concentration of carbon monoxide can be known by measuring this electromotive force. Here, the combustion exhaust gas passes through the gas selective permeable membrane 5 and reaches the solid electrolyte body 4, but the solid electrolyte body 4 has a pore size of 1
Since it is a porous film controlled to be 00 angstroms or less, a substance having a large molecular weight such as hydrocarbon cannot pass through the pores and does not reach the electrodes 2 and 3. The pore size of 100 angstroms or less is called a Knudsen diffusion region, and is a region in which the amount of gas passing through the pore is determined in proportion to the reciprocal of the square root of the molecular weight. Therefore, in this region, gas molecules having a larger molecular weight than carbon monoxide such as sulfurous acid gas can be prevented from substantially reaching the electrode.

In FIG. 2, reference numeral 8 is a catalyst supporting portion for supporting the carbon monoxide oxidation catalyst 1 in the pores of the gas selective permeable membrane 5 facing the electrode 2. With the above configuration, for example, when the combustion exhaust gas reaches the electrode 2 on the surface of the solid electrolyte body 4, the carbon monoxide in the combustion exhaust gas is oxidized by the carbon monoxide oxidation catalyst 1 of the catalyst supporting portion 8 and reaches the electrode 2. Without
At the electrode 2, oxygen is ionized by the reaction represented by (Chemical formula 1). On the other hand, since the combustion exhaust gas reaches the electrode 3 while containing carbon monoxide, the electrode 3 undergoes two reactions represented by (Chemical Formula 1) and (Chemical Formula 2). Therefore, an electromotive force is generated between the electrodes 2 and 3, and the concentration of carbon monoxide can be known by measuring the electromotive force. Here, the carbon monoxide oxidation catalyst 1 is loaded in the pores of the gas selective permeable membrane 5 by a wet manufacturing method such as a sol-gel method, and the surface of the solid electrolyte body 4 is subjected to electron beam vapor deposition or ion sputtering. The platinum electrodes 2 and 3 are formed by a dry manufacturing method such as a method.

In FIG. 3, the gas selective permeable membrane 5 has a hollow cylindrical shape and has a solid electrolyte in which an electrode 2 covered with a carbon monoxide oxidation catalyst 1 and an electrode 3 not covered with a carbon monoxide oxidation catalyst 1 are arranged. The body 4 is arranged in the hollow cylinder of the gas selective permeable membrane 5, both ends are sealed (not shown), and the coil-shaped heater 9 is provided on the outer peripheral portion of the gas selective permeable membrane 5.

With the above structure, the gas selective permeable membrane 5 is formed by extrusion molding of ceramics, and the whole is heated by the coiled heater 9 on the outer peripheral portion. Since the shape of the gas selective permeable membrane 5 is cylindrical, efficient heating can be performed, and a gas sensor with low power consumption can be realized. Further, by further forming the solid electrolyte body 4 in the hollow portion into a columnar shape, further miniaturization and energy saving can be realized.

In FIG. 4, 10 is a voltmeter for measuring an electromotive force between the electrodes 2 and 3, 11 is a voltage source for applying a voltage between the electrodes 2 and 3, 12 is an ammeter for measuring a circuit current, 1
Reference numeral 3 is a switching means for measuring electromotive force and measuring current. Microcomputer 1 gives instructions to switch between electromotive force measurement and current measurement.
4 is performed.

The current measurement will be described here. When a voltage is applied between the electrodes 2 and 3, oxygen ions move in the solid electrolyte body 4, and a current corresponding to the amount of movement is detected. Although the current value increases in proportion to the amount of movement of oxygen ions, it is limited by the flow rate of oxygen determined by the size of the pores of the gas selective permeable membrane 5 covering the solid electrolyte body 4.
That is, no matter how much voltage is applied, the current value shows a limiting current value defined by the flow rate of oxygen determined by the size of the pores of the gas selective permeable membrane 5 (referred to as diffusion-controlled). FIG. 5 shows the relationship between voltage and current. Since the limiting current value is proportional to the oxygen concentration in the combustion exhaust gas, it is possible to know the oxygen concentration in the combustion exhaust gas by detecting the limiting current value. a is a voltage-limit current characteristic at an oxygen concentration of 10% which is the upper limit of normal combustion. b is a voltage-limit current characteristic when the oxygen concentration is 5%, which is the lower limit of normal combustion. By detecting the magnitude of the limiting current in the combustion exhaust gas during the operation of the combustion device and comparing it with the limiting current range at the time of normal combustion stored in advance in the microcomputer 14, combustion is performed in a state of insufficient oxygen. When it is determined that normal combustion can be guaranteed by performing control such as increasing the rotation speed of the fan of the device. Since the oxygen concentration in the exhaust gas of a fan heater or the like is set to 5% to 10%, it is possible to guarantee the normal combustion operation by controlling the rotation speed of the fan so that it falls within this range.

Further, by detecting the limiting current value, it is possible to judge the conditions of the solid electrolyte body 4 and the gas selective permeable membrane 5. 6 and 4, when the combustion is stopped, the current value is measured in an environment in which the oxygen concentration in the air is constant while the current measurement is switched by the microcomputer 14. If the limiting current value a when the sensor is operating normally is known inside the microcomputer 14,
If only a current smaller than the current value can be detected, b, it can be determined that the pores of the gas selective permeable membrane 5 are clogged with tar or the like and the sensor is not operating normally. Further, when a current larger than the normal current value is detected c, it can be determined that the gas-selective permeable membrane 5 is damaged because oxygen flows too much in the gas-selective permeable membrane 5. When the abnormality of the sensor is detected, a signal is output from the microcomputer 14 to the notification means 15 or the like,
Signal a danger.

In FIG. 7, the heating means 6 near the gas selective permeable membrane 5 is larger than the heating voltage source 16 for supplying a voltage to the heating means 6, the first limiting resistor 17, and the first limiting resistor 17. In the second limiting resistor 18 having a resistance value, the resistance switching means 19 for switching the resistance connected in series with the heating means 6 to the first limiting resistance 17 or the second limiting resistance 18 according to an instruction from the microcomputer 14. is there. When the heating voltage source 16 applies a voltage to the heating means 6 via the second limiting resistance 18 by the resistance switching means 19 according to an instruction from the microcomputer 14, the side heat temperature at the time of carbon monoxide detection is maintained. On the other hand, when the resistance switching means 19 causes the heating voltage source 16 to apply a voltage to the heating means 6 via the first limiting resistor 17, the current value flowing through the heating means 6 increases and the temperature of the heating means 6 rises. . Therefore, even if the pores of the gas selective permeable membrane 5 are clogged with tar or the like, it can be burnt out by the heating means 6.

[0029]

As is apparent from the above description of the embodiments, the gas sensor of the present invention has a gas selective permeable membrane having a pore size of 10 mm.
Since the porous film is controlled to have a thickness of 0 angstrom or less, it is possible to prevent the substance that inhibits the carbon monoxide detection reaction and the gas that poisons the electrode from reaching the electrode, and improve the reliability and durability of the gas sensor. Has the effect.

Further, the solid electrolyte body and the electrode part are manufactured by a dry manufacturing method, the gas selective permeable membrane part is manufactured by a wet manufacturing method, and the sensor parts are manufactured by different manufacturing methods, and finally assembled and integrated. By doing so, the manufacturing process can be simplified, the yield can be improved, and the reliability can be improved.

Further, by forming the gas selective permeable membrane in a cylindrical shape and installing the solid electrolyte body and the electrode portion in the hollow portion,
It is possible to realize a gas sensor that is compact and saves energy.

Further, by applying a voltage between the electrodes of the solid electrolyte body and measuring the limiting current to judge whether or not the voltage is within the normal voltage range, the gas selective permeable membrane is damaged, and the pores of the pores due to tar or the like are observed. It is possible to realize a gas sensor that has a self-diagnosis function of the sensor itself, which is the key to detecting and controlling clogging, and realize a safe combustion device.

Further, when the oxygen concentration in the combustion exhaust gas is known by detecting the limiting current and it is determined that combustion is being performed in an oxygen-deficient state, the rotation speed of the fan of the combustion equipment is increased or combustion is stopped. The normal combustion can be guaranteed by controlling the above.

Further, by detecting the limiting current value, not only the oxygen concentration can be known, but also the normal or abnormal operation of the sensor can be detected. Therefore, the self-abnormality diagnosis of the gas sensor can be performed, thus improving the safety of the combustion equipment. It has the effect of being able to

Further, in order to guarantee the operation of the gas sensor, the temperature of the heating means in the vicinity of the gas selective permeable film is raised, so that the clogging of the gas selective permeable film is burned off, and the characteristics of the gas sensor can be recovered and the deterioration can be prevented.

[Brief description of drawings]

FIG. 1 is a sectional view of a gas sensor according to a first embodiment of the present invention.

FIG. 2 is a sectional view of a gas sensor according to a second embodiment of the present invention.

FIG. 3 is a sectional view of a gas sensor according to a third embodiment of the present invention.

FIG. 4 is a block diagram of a fourth embodiment of the present invention.

FIG. 5 is a voltage-current characteristic diagram of the fourth embodiment.

FIG. 6 is another voltage-current characteristic diagram of the fifth embodiment.

FIG. 7 is a block diagram of the sixth embodiment.

FIG. 8 is a sectional view of a conventional gas sensor.

[Explanation of symbols]

 1 Carbon Monoxide Oxidation Catalyst 2 Electrode 3 Electrode 4 Solid Electrolyte Body 5 Gas Selective Permeation Membrane 6 Heating Means 8 Catalyst Supporting Section 9 Heater 11 Voltage Source 12 Ammeter 13 Switching Means 14 Microcomputer 15 Notification Means 16 Heating Voltage Source 17 No. First limiting resistance 18 Second limiting resistance 19 Resistance switching means

Claims (6)

[Claims] 1. A solid electrolyte body having oxygen ion conductivity, comprising a pair of electrodes, an electrode covered with a catalyst capable of oxidizing carbon monoxide and an electrode not covered with the catalyst, and A gas sensor having a porous gas selective permeable membrane having a pore diameter of 100 angstroms or less, which covers a pair of electrodes, and a heating means in the vicinity of the gas selective permeable membrane. 2. A solid electrolyte body having oxygen ion conductivity having electrodes on the front and back surfaces, and a cover for covering the pair of electrodes.
Porous gas selective permeable membrane having a pore diameter of 00 angstroms or less, and a catalyst having the ability to oxidize carbon monoxide in the pores of the gas selective permeable membrane facing one of the pair of electrodes. And a heating means for supporting the gas, further comprising heating means in the vicinity of the gas selective permeable membrane. 3. A porous gas selective permeable membrane having a hole diameter of 100 angstroms or less and formed in a hollow column shape, an electrode covered with a catalyst having an ability to oxidize carbon monoxide, and a catalyst covered with the catalyst. A solid electrolyte body having oxygen ion conductivity in which a pair of electrodes with a non-electrode is arranged inside the hollow cylindrical gas selective permeable membrane with both ends sealed, and a coiled heater is provided on the outer peripheral portion of the gas selective permeable membrane. Gas sensor equipped. 4. A solid electrolyte body having oxygen ion conductivity, comprising a pair of electrodes, an electrode covered with a catalyst having an ability to oxidize carbon monoxide and an electrode not covered with the catalyst, and the pair. A gas sensor having a porous gas selective permeable membrane having a pore diameter of 100 angstroms or less and covering the electrodes, and a heating means near the gas selective permeable membrane, and an electromotive force measuring means between the pair of electrodes, A gas detection method comprising: a voltage applying means between the pair of electrodes; a current measuring means for measuring a current between the pair of electrodes; and a switching means for switching between the electromotive force measuring means and the current measuring means. 5. A solid electrolyte body having oxygen ion conductivity, comprising a pair of electrodes, an electrode covered with a catalyst having an ability to oxidize carbon monoxide and an electrode not covered with the catalyst, and the pair. A porous gas selective permeable membrane having a pore diameter of 100 angstroms or less, which covers the electrode, heating means in the vicinity of the selective permeable membrane, electromotive force measuring means between the pair of electrodes, and between the pair of electrodes. Voltage applying means, current measuring means for measuring the current between the pair of electrodes, switching means between the electromotive force measuring means and the current measuring means, and a self-determining abnormality of the gas sensor from the output of the current measuring means. A gas detection method provided with a diagnostic means. 6. A solid electrolyte body having oxygen ion conductivity, comprising an electrode covered with a catalyst capable of oxidizing carbon monoxide, a pair of electrodes not covered with the catalyst, and a solid electrolyte body having oxygen ion conductivity, and covering the pair of electrodes. A gas detection method comprising: a porous gas-selective permeable membrane having a pore diameter of 100 angstroms or less; heating means in the vicinity of the gas-selective permeable membrane; and a heating temperature control means for the heating means.